pthread_sigmask, pthread_kill, sigwait - handling of signals in threads
int pthread_sigmask(int how, const sigset_t *newmask, sigset_t *oldmask);
int pthread_kill(pthread_t thread, int signo);
int sigwait(const sigset_t *set, int *sig);
pthread_sigmask changes the signal mask for the calling thread as described by the how and
newmask arguments. If oldmask is not NULL, the previous signal mask is stored in the loca-
tion pointed to by oldmask.
The meaning of the how and newmask arguments is the same as for sigprocmask(2). If how is
SIG_SETMASK, the signal mask is set to newmask. If how is SIG_BLOCK, the signals speci-
fied to newmask are added to the current signal mask. If how is SIG_UNBLOCK, the signals
specified to newmask are removed from the current signal mask.
Recall that signal masks are set on a per-thread basis, but signal actions and signal han-
dlers, as set with sigaction(2), are shared between all threads.
pthread_kill send signal number signo to the thread thread. The signal is delivered and
handled as described in kill(2).
sigwait suspends the calling thread until one of the signals in set is delivered to the
calling thread. It then stores the number of the signal received in the location pointed
to by sig and returns. The signals in set must be blocked and not ignored on entrance to
sigwait. If the delivered signal has a signal handler function attached, that function is
sigwait is a cancellation point.
On success, 0 is returned. On failure, a non-zero error code is returned.
The pthread_sigmask function returns the following error codes on error:
EINVAL how is not one of SIG_SETMASK, SIG_BLOCK, or SIG_UNBLOCK
EFAULT newmask or oldmask point to invalid addresses
The pthread_kill function returns the following error codes on error:
EINVAL signo is not a valid signal number
ESRCH the thread thread does not exist (e.g. it has already terminated)
The sigwait function never returns an error.
Xavier Leroy <Xavier.Leroy@inria.fr>
sigprocmask(2), kill(2), sigaction(2), sigsuspend(2).
For sigwait to work reliably, the signals being waited for must be blocked in all threads,
not only in the calling thread, since otherwise the POSIX semantics for signal delivery do
not guarantee that it's the thread doing the sigwait that will receive the signal. The
best way to achieve this is block those signals before any threads are created, and never
unblock them in the program other than by calling sigwait.
Signal handling in LinuxThreads departs significantly from the POSIX standard. According
to the standard, ``asynchronous'' (external) signals are addressed to the whole process
(the collection of all threads), which then delivers them to one particular thread. The
thread that actually receives the signal is any thread that does not currently block the
In LinuxThreads, each thread is actually a kernel process with its own PID, so external
signals are always directed to one particular thread. If, for instance, another thread is
blocked in sigwait on that signal, it will not be restarted.
The LinuxThreads implementation of sigwait installs dummy signal handlers for the signals
in set for the duration of the wait. Since signal handlers are shared between all threads,
other threads must not attach their own signal handlers to these signals, or alternatively
they should all block these signals (which is recommended anyway -- see the Notes sec-